| Document And Entity Information | Apr. 08, 2020 |
| Document Information [Line Items] | |
| Entity, Registrant Name | ICF International, Inc. |
| Document, Type | 8-K |
| Document, Period End Date | Apr. 8, 2020 |
| Entity, Incorporation, State or Country Code | DE |
| Entity, File Number | 001-33045 |
| Entity, Tax Identification Number | 22-3661438 |
| Entity, Address, Address Line One | 9300 Lee Highway |
| Entity, Address, City or Town | Fairfax |
| Entity, Address, State or Province | VA |
| Entity, Address, Postal Zip Code | 22031 |
| City Area Code | 703 |
| Local Phone Number | 934-3000 |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | ICFI |
| Security Exchange Name | NASDAQ |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity, Emerging Growth Company | false |
| Amendment Flag | false |
| Entity, Central Index Key | 0001362004 |
